In 1994 we discovered that yeast can have prions, infectious proteins analogous to the transmissible spongiform encephalopathies of mammals. We showed that the non-mendelian genetic element, URE3, is a prion of the Ure2 protein, and that PSI+ is a prion of Sup35p (1,). These prions are amyloids of the respective proteins (3). Our discovery showed that proteins can be genes. Unexpectedly, shuffling the prion domain amino acid sequence of Ure2p or Sup35p did not alter the ability of these domains to support prion formation, suggesting that the amyloid structure is parallel in-register (4). We have shown by solid-state NMR (with Rob Tycko of NIDDK) that the amyloids of Ure2p, Sup35p and Rnq1p are indeed in-register parallel beta sheets (5-7). It has not escaped our notice that this in-register parallel beta sheet structure can explain how a given protein sequence can encode any of several biologically distinct prion variants based on biochemically distinct self-propagating amyloid structures (8). We demonstrated URE3 prions based on Ure2 proteins from non-cerevisiae species of Saccharomyces, and found a similar species barrier to that seen among mammals of different species in their transmission of spongiform encephalopathies (9). We showed that the variant properties, as defined by species barrier, are maintained even during passage through a different species. We also noted that the Ure2p of Saccharomyces castellii cannot become a prion (9). The Candida albicans Ure2p can form a URE3 prion in S. cerevisiae (10), or in C. albicans itself, engineered to detect this prion (11), but the corresponding domain of Candida glabrata cannot form a prion, in either S. cerevisiae (10) or in C. albicans (11), even though the prion domain of glabrata is closer in sequence to that of cerevisiae than is that of albicans. Thus the conservation of sequence in the Ure2 prion domains is not for prion-forming ability, but reflects the function of this domain in protecting the full length protein from degradation in vivo (12). The C. albicans Ure2 protein or its prion domain each readily form amyloid which is highly infectious for yeast, and, like the other yeast prions, has a parallel in-register beta sheet architecture (13). We find that PSI+ is rare in wild strains, though it would be common if it were advantageous (13). We used population genetics to quantify the detriment to cells of carrying the yeast prions PSI+, URE3 and PIN+ (14). From the known detriment of carrying the 2 micron DNA plasmid, determined by three different groups, and the incidence of 2 micron DNA in wild strains (13), we could infer the frequency of outcross mating by S. cerevisiae, 1000 times more often than had been previously believed (14). Using this information, and the rarity of the yeast prions in wild strains (13), we showed that even the mildest variants of each of these prions must confer at least a 1% growth/survival defect (14). Is PSI+ always so mild? We designed a method to find lethal (Suicidal) PSI+s, should they exist. We found that such lethal or near-lethal variants of PSI+ comprise more than half of total isolates (15). We found that common variants of the URE3 prions cause extremely slow growth, although deletion of the URE2 gene in these strains did not slow growth (15). This toxic URE3 must be a due to a pathogenic amyloid,confirming the pathologic nature of the yeast prions PSI+ and URE3. Understanding their mechanisms of pathogenesis may be useful in understanding human amyloidoses. We have sequenced the SUP35 genes of 55 wild S. cerevisiae isolates, finding three groups of common polymorphs (16). PSI+ transmission between polymorphs is largely blocked, suggesting that these changes are selected to protect yeast from the detrimental effects of the prion (16). Indeed, the rate of evolutionary change of the prion domain is much faster than that of the remainder of the molecule suggesting that selection for resistance to infection by prions is driving change in the prion domain. We find that the rare wild PSI+ variants are sensitive to these blocks as well, supporting this interpretation (17). M domain changes in one polymorph are important in blocking prion spread (16). We find that transmission efficiency of PSI+ from a strain with one Sup35p polymorph to one with another polymorph is highly dependent on the variant of PSI+ (16, 17). That is, two PSI+ isolates in the otherwise identical genetic background with the same Sup35p protein sequence can have dramatically different efficiencies of transmission to cells with a different Sup35 protein sequence. Using this transmission frequency as a marker for different prion variants, we demonstrated segregation (separation) of prion variants during growth of the cells under non-selective conditions, and the generation of new variants, presumably due to occasional mis-templating of the amyloid (17). Data suggestive of this prion cloud phenomenon in mammalian prions has been published and it is likely to apply as well to the common human amyloid diseases. Our structural studies of amyloid of transthyretin showed that it is unique among characterized pathologic amyloids in not having the in-register parallel beta sheet structure (18). 1. Wickner RB (1994) URE3 as an altered URE2 protein: evidence for a prion analog in S. cerevisiae. Science 264: 566 - 569. 2. Masison DC & Wickner RB (1995) Science 270: 93 - 95. 3. Wickner RB, Edskes HK, Ross ED, Pierce MM, Baxa U, Brachmann A & Shewmaker F (2004) Ann. Rev. Genetics 38: 681-707. 4. Ross ED, Minton AP & Wickner RB (2005) Nature Cell Biol. 7: 1039-1044. 5. Shewmaker F, Wickner RB & Tycko R (2006) Proc. Natl. Acad. Sci. USA 103: 19754 - 19759. 6. Baxa U, Wickner RB, Steven AC, Anderson D, Marekov L, Yau W-M & Tycko R (2007) Biochemistry 46: 13149 - 13162. 7. Wickner RB, Dyda F & Tycko R (2008) Proc Natl Acad Sci U S A 105: 2403 - 2408. 8. Wickner RB, Shewmaker F, Kryndushkin D & Edskes HK (2008) Protein inheritance (prions) based on parallel in-register beta-sheet amyloid structures. Bioessays 30: 955 - 964. 9. Edskes HK, McCann LM, Hebert AM & Wickner RB (2009) Prion variants and species barriers among Saccharomyces Ure2 proteins. Genetics 181: 1159 - 1167. 10. 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Proc Natl Acad Sci USA 108:5337 - 41. 16. Bateman, DA and Wickner, RB (2013) PSI+ Prion transmission barriers protect Saccharomyces cerevisiae from infection: intraspecies 'Species Barriers'. Genetics 190:569-579. 17. Bateman, D., and Wickner, R. B. (2013) The PSI+ prion exists as a dynamic cloud of variants, Plos Genet. 9(1):e1003257. 18. Bateman, DA, Tycko, R and Wickner, RB (2011) Experimentally derived structural constraints for amyloid fibrils of wild-type transthyretin. Biophys. J. 101:2485-2492.